Julian Date Converter

Convert any Gregorian calendar date into its Julian Day Number (JDN), the continuous day count astronomers and satellite operators rely on for precise chronology.

Use the full four-digit year; astronomical year numbering handles BCE dates with zero and negatives.
Enter 1–12 for January through December.
Provide the day of month in the Gregorian calendar.

Applies the proleptic Gregorian calendar. For historical research before regional adoption, confirm the source calendar (Julian vs. Gregorian) before converting.

Examples

  • Gregorian 2000‑01‑01 ⇒ JDN 2,451,545, the widely used J2000.0 epoch
  • Partial eclipse on 2024‑04‑08 ⇒ JDN 2,460,393
  • Leap day 2032‑02‑29 ⇒ JDN 2,459,674

FAQ

What is the difference between JDN and Julian calendar dates?

Julian Day Number is a continuous count used in astronomy, whereas the Julian calendar is a historical civil calendar. This converter returns the astronomical JDN.

Can I enter BCE years?

Yes. Use astronomical year numbering where 0 corresponds to 1 BCE, −1 to 2 BCE, and so on.

How do I include the time of day?

Compute the JDN for the date and add (hour − 12)/24 + minute/1,440 + second/86,400 to get the fractional component.

Why does the result include large numbers?

JDN counts days since 4713 BCE, so modern dates exceed two million. The high value simplifies interval math across millennia.

Additional Information

  • Formula implements the proleptic Gregorian calendar back to 4713 BCE, following the standard astronomical algorithm.
  • Outputs the noon-based Julian Day Number (starting at 12:00 UTC). Add fractional days separately if you need time-of-day precision.
  • Useful for calculating elapsed days between observations, scheduling space missions, or indexing archival datasets.

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